1. Field of the Invention
The invention relates to a headbox for a machine for producing a fibrous web, in particular a paper or cardboard web from at least one fibrous stock suspension, comprising a feed device feeding the at least one fibrous stock suspension, a perforated distribution pipe plate arranged immediately downstream thereof and having a plurality of channels arranged in rows and columns, an intermediate channel arranged downstream thereof, extending over the width of the headbox and having a plurality of means for dosing of a fluid in partial fluid streams to the at least one fibrous stock suspension in a preferably adjustable/controllable manner, the means being spaced apart from each other in width direction of the headbox and the individual means comprising a plurality of dosing channels having respective dosing channel openings on the outlet side and a dosing channel length, discharging at different levels and being connected to a common supply channel. The headbox further comprises a downstream turbulence generator having a plurality of flow channels arranged in rows and columns and a headbox nozzle comprising a nozzle gap, located immediately adjacent to the turbulence generator.
2. Description of the Related Art
A headbox of this type is known for example from German disclosure documents DE44 16 898 A1, DE 44 16 899 A1 and DE 44 16 909 A1.
German disclosure document DE 44 16 898 A1 describes a headbox for a paper machine; including a feed device for the stock suspension comprising a guiding element with a plurality of channels; upstream from a guiding element a mixing chamber with several infeed devices which are distributed over the headbox for a fluid which is to be added to the stock suspension and which differs in its characteristics from the stock suspension; and with a nozzle chamber forming an outlet opening for the stock suspension, located immediately downstream from the guiding element. The feed devices for the fluid which is to be added extend essentially vertically in the mixing chamber and comprise several dosing openings located on top of each other.
German disclosure document DE 44 16 899 A1 describes a headbox for a paper machine including a feed device for the stock suspension, with a first guiding element comprising a plurality of channels, a second guiding element comprising a plurality of channels, an intermediate mixing chamber, a contiguous nozzle chamber forming an outlet opening for the stock suspension, and several feed lines distributed over the headbox for the fluid which is to be added. The majority of the channels of the guiding elements are positioned and dimensioned so that the outlet opening of the upstream guiding element is not aligned with a confluence into the downstream guiding element. The feed lines for the fluid which is to be added are equipped with dosing openings, the majority of which align respectively with a confluence into the downstream guiding element.
In addition, German disclosure document DE 44 16 909 A1 describes a headbox for a paper machine with a feed device for the stock suspension, a downstream contiguous guiding element equipped with a plurality of channels, located downstream from there a downstream mixing chamber extending across the width of the headbox with several separation walls distributed across the headbox and extending essentially in flow direction, as well as feed devices for a fluid which is to be added, distributed in cross direction of the headbox and which are equipped with dosing openings, downstream an additional guiding element equipped with a plurality of channels and adjacent to it, one nozzle chamber forming an outlet opening for the stock suspension.
The feed devices for the fluid which is to be added extend into the mixing chamber and the sectional area which is formed in the mixing chamber by the separation walls begins in the region of the dosing openings from where it extends essentially in flow direction to the second guiding element.
The headboxes described in these three documents provide systems which are able to fulfill especially the technological requirements regarding good blending quality of fibrous stock suspension and fluid, sufficient volume stream consistency, low effective width and a low mapping offset more or less well, or only partially. The blending quality of fibrous stock suspension and fluid is usually positively influenced by good pre-distribution of the mediums in height direction, by high turbulence intensity in the mediums and by a long distance over which blending of the mediums occurs. A low dynamic at the dosing location and a high pressure value after the dosing location are favorable for volume flow consistency. An adjusted and steadied flow before the dosing location, light cross flows at the dosing location and only a short, or preferably no intermediate channel after the dosing location, a synchronous separation of dosing location and turbulence generator and a targeted locally stable dosing into the turbulence generator are important for the desired small effective width. Generally the same criterion applies for the mapping offset requirements as for the effective width requirements. However, there is the additional criterion of a good volume flow constancy and slight cross compensation in the headbox.
It is therefore the objective of the current invention, and what is needed in the art is, to improve a headbox of the type described at the beginning, so that the known disadvantages of the state of the art are significantly, preferably completely eliminated and that a substantial improvement in meeting especially the technological requirements is achieved. In addition, a technically simple and reliable dosing system is to be cited which can be implemented in an economical and technically simple manner.